Point defects in synthetic diamond material, particularly quantum spin defects and/or optically active defects, have been proposed for use in various sensing, detecting, and quantum processing applications including: magnetometers; spin resonance devices such as nuclear magnetic resonance (NMR) and electron spin resonance (ESR) devices; spin resonance imaging devices for magnetic resonance imaging (MRI); and quantum information processing devices such as for quantum computing.
Many point defects have been studied in synthetic diamond material including: silicon containing defects such as silicon-vacancy defects (Si-V), silicon di-vacancy defects (Si-V2), silicon-vacancy-hydrogen defects (Si-V:H), silicon di-vacancy hydrogen defects (S-V2:H); nickel containing defect; chromium containing defects; and nitrogen containing defects such as nitrogen-vacancy defects (N-V), di-nitrogen vacancy defects (N-V-N), and nitrogen-vacancy-hydrogen defects (N-V-H). These defects are typically found in a neutral charge state or in a negative charge state. It will be noted that these point defects extend over more than one crystal lattice point. The term point defect as used herein is intended to encompass such defects but not include larger cluster defects, such as those extending over ten or more lattice points, or extended defects such as dislocations which may extend over many lattice points.
It has been found that certain defects in synthetic diamond material are particularly useful for sensing, detecting, and quantum processing applications. For example, the negatively charged nitrogen-vacancy defect (NV−) in synthetic diamond material has attracted a lot of interest as a useful quantum spin defect because it has several desirable features including:                (i) Its electron spin states can be coherently manipulated with high fidelity owing to an extremely long coherence time (which may be quantified and compared using the transverse relaxation time T2);        (ii) Its electronic structure allows the defect to be optically pumped into its electronic ground state allowing such defects to be placed into a specific electronic spin state even at non-cryogenic temperatures. This can negate the requirement for expensive and bulky cryogenic cooling apparatus for certain applications where miniaturization is desired. Furthermore, the defect can function as a source of photons which all have the same spin state; and        (iii) Its electronic structure comprises emissive and non-emissive electron spin states which allows the electron spin state of the defect to be read out through photons. This is convenient for reading out information from synthetic diamond material used in sensing applications such as magnetometry, spin resonance spectroscopy and imaging. Furthermore, it is a key ingredient towards using the NV− defects as qubits for long-distance quantum communications and scalable quantum computation. Such results make the NV− defect a competitive candidate for solid-state quantum information processing (QIP).        
The NV− defect in diamond consists of a substitutional nitrogen atom adjacent to a carbon vacancy as shown in FIG. 1a. Its two unpaired electrons form a spin triplet in the electronic ground state (3A), the degenerate ms=±1 sublevels being separated from the ms=0 level by 2.87 GHz. The electronic structure of the NV− defect is illustrated in FIG. 1b from Steingert et al. “High sensitivity magnetic imaging using an array of spins in diamond”, Review of Scientific Instruments 81, 043705 (2010). The ms=0 sublevel exhibits a high fluorescence rate when optically pumped, for example using a 532 nm laser. In contrast, when the defect is excited in the ms=±1 levels, it exhibits a higher probability to cross over to the non-radiative singlet state (1A) followed by a subsequent relaxation into ms=0. As a result, the spin state can be optically read out, the ms=0 state being “bright” and the ms=±1 states being dark. When an external magnetic field is applied, the degeneracy of the spin sublevels ms=±1 is broken via Zeeman splitting. This causes the resonance lines to split depending on the applied magnetic field magnitude and its direction. This dependency can be used for vector magnetometry as the resonant spin transitions can be probed by sweeping the microwave (MW) frequency resulting in characteristic dips in the optically detected magnetic resonance (ODMR) spectrum as shown in FIG. 2a from Steinert et al.
Steinert et al. employed ion implantation to create a homogenous layer of negatively charged NV− centres into an ultrapure {100} type IIa diamond. The ensemble NV− sensor was found to offer a higher magnetic sensitivity due to the amplified fluorescence signal from a plurality of sensing spins. Another option is vector reconstruction since the diamond lattice imposes four distinct tetrahedral NV− orientations as shown in FIG. 2b from Steinert et al. The magnetic field projections along each of these axes can be measured as a single composite spectrum and a numerical algorithm used to reconstruct the full magnetic field vector. The magnitude (B) and orientation (θB, φB) of the external magnetic field can be calculated by analyzing the ODMR spectra based on an unconstrained least-square algorithm.
One major problem in producing materials suitable for quantum applications is preventing quantum spin defects from decohering, or at least lengthening the time a system takes to decohere (i.e. lengthening the “decoherence time”). A long T2 time is desirable in applications such as quantum computing as it allows more time for the operation of an array of quantum gates and thus allows more complex quantum computations to be performed. A long T2 time is also desirable for increasing sensitivity to changes in the electric and magnetic environment in sensing applications.
Kennedy et al. have disclosed that the decoherence time of NV− defects in synthetic CVD (chemical vapour deposited) diamond material is longer than for NV− defects in synthetic HPHT (high pressure high temperature) diamond material and that low nitrogen concentration in synthetic CVD diamond material is a factor in achieving longer decoherence times (see, for example, Phys. Stat. Sol. (b) 233, no. 3, 416-426 (2002) and Appl. Phys. Lett. vol. 83, no. 20, 4190-4192 (2003)). Kennedy et al. disclose an NV− defect decoherence time of 58 μs at room temperature (300 K) for a CVD diamond material having a single substitutional nitrogen concentration of 30 ppb.
Subsequently, through careful use and control of various manufacturing techniques Scarsbrook et al. have fabricated a single crystal CVD diamond material with NV− defects having a decoherence time greater than 600 μs (see, for example, WO 2010010344 and WO 2010010352).
WO 2010010344 discloses that single crystal synthetic CVD diamond material which has a high chemical purity, i.e. a low nitrogen content, and wherein a surface of the diamond material has been processed to minimise the presence of crystal defects, can be used to form a solid state system comprising a quantum spin defect. Where such materials are used as a host for quantum spin defects, long T2 times are obtained at room temperature and the frequency of the optical transitions used to read/write to devices are stable.
WO 2010010352 discloses that by carefully controlling the conditions under which single crystal synthetic CVD diamond material is prepared, it is possible to provide synthetic diamond material which combines a very high chemical purity, a very high crystallographic purity, and a very high isotopic purity. By controlling the chemical purity, crystallographic purity, and isotopic purity of the material used in the CVD process, it is possible to obtain synthetic diamond material which is particularly suitable for use as a host for a quantum spin defect. Where such materials are used as a host for quantum spin defects, long T2 times are obtained at room temperature and the frequency of the optical transitions used to read/write to the devices are stable. A layer of synthetic CVD diamond material is disclosed which has a low nitrogen concentration and a low concentration of 13C. The layer of synthetic diamond material has very low impurity levels and very low associated point defect levels. In addition, the layer of synthetic CVD diamond material has a low dislocation density, low strain, and vacancy and self-interstitial concentrations which are sufficiently close to thermodynamic values associated with the growth temperature that its optical absorption is essentially that of a perfect diamond lattice.
In light of the above, it is evident that WO 2010010344 and WO 2010010352 disclose methods of manufacturing very high quality “quantum grade” single crystal synthetic CVD diamond material. The term “quantum grade” is used herein for diamond material which is suitable for use in applications that utilize the material's quantum spin properties. Specifically, the quantum grade diamond material's high purity makes it possible to isolate single defect centres using optical techniques known to the person skilled in the art. The term “quantum diamond material” is also used to refer to such material.
One problem with quantum materials is that single photon emission from quantum spin defects in such materials can be very weak. For example, NV− defects in diamond exhibit a broad spectral emission associated with a Debye-Waller factor of the order of 0.05, even at low temperature. Emission of single photons in the Zero-Phonon Line (ZPL) is then extremely weak, typically of the order of a few thousands of photons per second. Such counting rates might be insufficient for the realization of advanced QIP protocols based on coupling between spin states and optical transitions within reasonable data acquisition times.
The problem of weak emission may be alleviated to some extent by increasing the number of quantum spin defects such that a large number of emitting species exists in the material. For example, the number of NV− defects may be increased by increasing the concentration of nitrogen in the diamond material. Synthetic CVD diamond material can be grown with >10 ppm nitrogen. However, the presence of nitrogen at high concentrations in the CVD growth process typically results in the incorporation of other defects which: (i) leads to an increase in diamond absorption affecting the efficiency of excitation of NV− defects and light collection therefrom; and (ii) leads to a reduction in the decoherence time due to decoherence mechanisms linked to other defects. While these other defects can be reduced by application of appropriate high temperature annealing techniques, in some applications this is undesirable due to residual graphitization and other potential complications.
In light of the above, while the longest decoherence times are reported for very low defect synthetic CVD diamond materials, the sensitivity these synthetic CVD diamond materials offer are compromised by the reduced NV− defect concentration. Furthermore, methods to increase the NV− defect concentration through implantation have limited value due to residual spin defects introduced due to implantation damage.
It is an aim of certain embodiments of the present invention to at least partially solve one or more of the aforementioned problems.
In relation to the above, US2006/0234419 and U.S. Pat. No. 6,582,513 disclose that CVD diamond can be grown with layers of controlled purity and thickness. It is further disclosed that since the number of atoms of nitrogen in a diamond film will be a function of concentration and thickness, NV− defects may be isolated from other defects. In other words, given a known concentration of NV defects that will be formed in a given volume of CVD grown diamond, making the diamond layer very thin assures that very few NV defects are formed, and are thus isolated from each other. WO2007/009037 also discloses that the isolation of NV defects is a function of thickness and nitrogen content and that a thin layer of CVD diamond material can provide isolated NV defects. Implantation and release is disclosed as a means for separating a thin as-grown layer of CVD diamond material from a substrate on which it is grown. Such an implantation and release mechanism for separating thin films of as-grown CVD diamond material from a substrate is also disclosed in US2005/0181210.